A typical inkjet printer uses one or more printheads to form an ink image on an image receiving surface. Each printhead typically contains an array of individual inkjets for ejecting drops of ink across an open gap to the image receiving surface to form an ink image. The image receiving surface may be on a continuous web of recording media, a series of media sheets, or on a rotating image receiving member, such as a print drum or endless belt. Images printed on a rotating surface are later transferred to recording media by mechanical force in a transfix nip formed by the rotating member and a transfix roller. In an inkjet printhead, the inkjets include individual piezoelectric, thermal, or acoustic actuators that generate mechanical forces to expel ink through a nozzle from an ink filled pressure chamber in response to an electrical voltage signal, sometimes called a firing signal. The magnitude, frequency, and/or duration of the firing signals affect the amount of ink ejected in each drop. The firing signal is generated by a printhead controller with reference to digital image data. An inkjet printer generates firing signals with reference to the digital image data to operate the inkjets and form an ink image with individual ink drops at particular locations on the image receiving surface. The locations where the ink drops landed are sometimes called “ink drop locations,” “ink drop positions,” or “pixels.” Thus, a printing operation can be viewed as the placement of ink drops on an image receiving surface with reference to digital image data.
Modern inkjet printers typically receive digital image data in a variety of formats and form ink images on a print medium that reproduce the original digital image. One challenge in forming printed images includes the accurate reproduction of colors in a physically printed image from the digital image data. For example, the digital image data are often not encoded in a format that is directly suitable for operating inkjets in the printer. Instead, the digital image data are often encoded in either a device-independent color space, such as L*a*b*, or are encoded in a digital format such as the red, green, blue (RGB) that is associated with display screens rather than printed images. The printer or an intermediate computing device converts the digital image data into a data format that corresponds to the ink colors that are available in the printer. One type of printer forms images using cyan, magenta, yellow, and black (CMYK) inks. The printer reproduces a wide range of colors by interspersing small drops of the basic CMYK colors on the print medium. The human eye perceives different colors from the combination of the CMYK colors. Existing standards, which include profiles from the International Color Consortium (ICC), are used to covert image data from an input color space into a color space that is used to eject ink drops from the inkjets to form the printed images.
Existing printers can print a wide range of colors in printed images using a small number of inks such as CMYK or other combinations of ink colors. A range of colors that can be represented in a color space are referred to as a “gamut” of colors. Existing printers, however, can have difficulty in reproducing some colors within the gamut of the color space. For example, reproduction of some colors in a gamut require the printer to print so many ink drops onto a media sheet that the ink would oversaturate the media sheet or would interfere with other components in the printer during a printing process.
To help prevent oversaturation, existing printers identify a coverage parameter of the printed ink and limit the amount of ink that is formed on the print medium based on the coverage parameter. The term “coverage parameter” refers to a percentage or proportion of the print medium that is perceived as being covered by ink. For example, a 100% coverage indicates that an area of the print medium is perceived as being fully covered by one type of ink. The coverage parameter can exceed 100% when multiple ink colors are printed on the same area of a print medium, such as a 200% coverage, which could include, for example, a 50% coverage of the area for each of the CMYK inks. Various ink colors have different perceptibility levels. For example, black ink on white paper is typically more perceptible than an equivalent amount of yellow ink on the same white paper. Existing printers that limit the printed ink based on the coverage density parameter print images with a narrower color gamut, and cannot reproduce certain colors. Consequently, improvements to the operation of inkjet printers to generate printed images with a wider color gamut without interfering with the operation of the printer would be beneficial.